The United States publicly acknowledged on Monday that it conducts targeted killing of al- Qaeda operatives using unmanned aerial vehicles, also known as drones, arguing that the attacks are legal and efficient.

John Brennan, President Barack Obama's homeland security and counterterrorism adviser, publicly acknowledged the programs during a speech at the Woodrow Wilson International Center for Scholars, a Washington-based think tank.

"In full accordance with the law -- and in order to prevent terrorist attacks on the United States and to save American lives - - the United States government conducts targeted strikes against specific al-Qaeda terrorists, sometimes using remotely piloted aircraft, often referred to publicly as drones," said Brennan.

This is the first time a senior U.S. official has publicly acknowledged the practice, although it has been widely known and privately discussed by many.

Brennan said he made the announcement because "President Obama has instructed us to be more open with the American people about these efforts." He said the strikes are "legal," as the United States is "in an armed conflict with al-Qaeda, the Taliban, and associated forces, in response to the 9/11 attacks," and may "use force consistent with our inherent right of national self-defense. "

Brennan claimed the attacks are also ethical, necessary, and efficient. He said the administration has worked to refine, clarify, and strengthen the process of review and authorization of the attacks, and an individual must pose "a significant threat to U.S. interests" to be approved for targeted killing.

Drone attacks have significantly increased since Obama took office, becoming a contentious issue between the United States and Pakistan, where many of such attacks occurred. The method was also used in Afghanistan, Yemen and the Horn of Africa.